Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter of a temperature above 0 K. Thermal radiation represents a conversion thermal energy into electromagnetic energy and can be classified into infrared radiation, visible light and ultraviolet light according to the radiation wavelength or frequency. All matter with a temperature above 0 K is composed of atoms or molecules which are in random thermal motion and interact with each other. These atoms and molecules are composed of charged particles, e.g., protons and electrons, and kinetic interactions among these particles can generate coupled electrical and magnetic fields, resulting in energy emission in the form of electromagnetic waves or photons from the body surface. Examples of thermal radiation include the ultraviolet, visible and infrared light emitted by sun and the infrared radiation emitted by animals. Due to the significance of thermal radiation detection in numerous applications, various thermal radiation detection technologies have been developed, including the photovoltaic detectors, bolometers, pyroelectric detectors and thermoelectric detectors. Among these technologies, thermal radiation microsensors based on thermoelectric effects have been attracting great attentions in recent years due to their high stability, low cost for mass production, low power dissipation, wide working range, and the compatibility with microfabrication technology.
Conventional thermoelectric thermal radiation sensors generally consist of an absorbing plate (hot end) suspended by thermoelectric beams from the cold end. This type of design is called “planar design” because all the structures of the sensor are in the same plane and the heat flow direction is parallel to the plane. There are many disadvantage associated with the planar design. The fill factor of this type of design is very low, typically lower than 40%, because the thermoelectric legs take lots of space of the sensor area. Meanwhile, these planar microsensors are generally fabricated using photolithography techniques and the dimensions of each component are difficult to reduce below 1-2 microns. To suspend the thermal radiation absorption membrane, the thermopiles or the underlying supports must be thick enough to ensure the mechanical strength and their aspect ratios are normally small, which, however, limit the overall performance of thermoelectric thermal radiation sensors. Compared with other thermal radiation sensors such as bolometers and photovoltaic sensors whose responsivities are around 1000 V/W, planar thermoelectric thermal radiation sensors of a similar size are typically much less responsive (˜200 V/W). Therefore, they usually have larger sensor sizes to increase the signal output. State-of-the-art thermoelectric sensors can have responsivities similar to that of a typical bolometer sensor but their dimensions are much larger. Large sensor size also causes a slow thermal response of the sensor. It is difficult to achieve a high performance while keeping the sensing unit miniaturized. Therefore, the conventional planar design is not suitable to apply in applications requiring high spatial resolution or fast response such as high-resolution thermal imaging. Thermoelectric sensors with a high responsivity, small size and short response time are required for the potential application of thermoelectric technology in high-resolution or high-speed thermal radiation detections.
The above-described background relating to thermal radiation sensing for various applications is merely intended to provide a contextual overview of thermal radiation technology, and is not intended to be exhaustive. Other context regarding thermal radiation and microsensors may become further apparent upon review of the following detailed description.